3GPP Release 13 NB-IoT has triggered attention of research and operator communities, as one of the most appealing enablers of massive Machine Type Communication (mMTC) and cellular Internet of Things (cIoT). NB-IoT specific features, together with peculiarities of the implied novel and complex scenarios that involve an ever-increasing number of communicating devices, require extensive analyses and advanced testing, i.e. field trials and ad-hoc simulation experiments. To this aim, a NB-IoT simulator, based on the upgrade of the ns-3 LTE suite, is proposed and will be applied to a case study of NB-IoT network-wide analysis. Configuration of NB-IoT procedures of network access, coverage level estimate, and coverage enhancement via signal repetitions, together with a planned resource distribution with coexisting LTE/LTE-A systems, will be analysed in order to provide performance in terms of network and device efficiency.
The project will provide a significant contribution in the realization of a ns-3 based NB-IoT simulator, and will also contribute to the experimental testing of NB-IoT networks. Ns-3 is a C++-based, event-driven simulator, that implements several models of networks and standardized systems. The simulator is organized in modules, providing self-contained functions. Among others, a stable implementation of LTE is present, mainly developed within the LENA project [9], and capable to scale LTE simulations up to tens of eNBs and hundreds of UEs. As a matter of fact, ns-3 limits the number of deployable UEs in LTE simulation. This limit derives from the frequency with which UEs sound channel conditions toward eNBs vs. number of eNBs. Given a number of eNBs, and the largest possible refresh time for the sounding reference signal (SRS), i.e. 320 ms as specified in the 3GPP Specification TS 36.213 [10], the maximum number of UEs that can be included in the simulator is derived. The most important LENA modules toward NB-IoT simulation are propagation, spectrum, and LTE modules, that are further described below. The propagation module implements propagation and error models at the PHY level. Several channel models are included by default, such as Friis open space, Okamura-Hata, and models that take into account the presence of buildings and other obstructing objects, as contained in the 2015 ITU Recommendation P.1411-8 [11]. The spectrum module describes each transmitted packet as a physical signal with a specific power spectral density, making it possible to evaluate mutual interference among signals. The goal of propagation and spectrum modules is to provide a measure of the received signal power, along with thermal and interference noise. The receiver module converts then a received power into a block error rate (BLER), to evaluate if a packet is correctly received. The conversion takes into account signal modulation and coding, and it is performed via appropriate signal to interference vs. BLER curves, derived with low-level simulations (e.g., Matlab-based simulators implemented at Technische Universität Wien, Vienna, Austria). The LTE module implements LTE and Evolved Packet Core (EPC) models. The LTE model includes the overall radio protocol stack (PHY, MAC, and higher layers), supporting radio resource management, CQI evaluation, QoS-aware packet scheduling, and inter-cell interference coordination. Several parameters, such as antenna models, gains, and transmission power can be configured at the PHY level. At the MAC layer, resource allocation is handled, taking into account the effects of the AMC technique. At higher layers, TCP/UDP traffic protocols can be used. Different UEs may run different applications, with appropriate QoS specifications. The EPC model includes core network entities, such as Packet Data Network Gateway (PGW), Serving Gateway (SGW), and Mobility Management Entity (MME). It implements the end-to-end IP connectivity, and also supports the X2 interface-based handover between two eNBs, in case of UE mobility.
Starting from the ns-3 LTE suite, a NB-IoT simulation environment will be implemented, by including new functionalities into the LTE suite and adapting existing ones. The activities will focus on embedding the LTE suite with NB-IoT features that mostly differ from LTE/LTE-A, in particular synchronization and signaling procedures. Moreover, the simulator will be used to address network-wide analysis, by appropriately tuning ns-3 LTE parameters. Further refinement of the simulator will be focused on the introduction of specific NB-IoT RACH and radio resource control (RRC) protocols, as well as a PHY layer. The possible experimental campaigns will complement the simulator, and will possible trigger comparative analysis between simulation and experimental results on aspects related to:
" RACH Procedure and CL Estimate
" Topology and Coexistence with LTE/LTE-A
" CE and Throughput Performance
[9] N. Baldo, "The ns-3 LTE module by the LENA project," 2011. [Online]. Available: https://www2.nsnam.org/tutorials/tutorials/consortium13/lte-tutorial.pdf.
[10] 3GPP Spec. TS 36.213, "Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures," v. 13.0.0, Jan. 2016.
[11] ITU-T Rec. P.1411-8, "Propagation data and prediction methods for the planning of short-range outdoor radiocommunication systems and radio local area networks in the frequency range 300 MHz to 100 GHz," July 2015.